Digital modulation signal generator

ABSTRACT

With respect to a digital modulation signal outputted from an output terminal, in order to enable desired carrier leak characteristics to be compatible with desired mutual modulation distortion characteristics according to a measurement object, a first level varying member is provided between an amplifier and an output terminal. In addition, a second level varying member is provided between a base band signal generator and an orthogonal modulator. The first and second level varying members are respectively set to be predetermined attenuation quantity values based on an output level value of a digital modulation signal specified by an output level specifying member and the judgment result caused by a judgment portion.

This application is a U.S. National Phase Application under 35 USC 371of International Application PCT/JP00/07660 (not published in English)filed Oct. 31, 2000.

BACKGROUND OF THE INVENTION

The present invention pertains to a digital modulation signal generatingapparatus. In particular, the present invention relates to a digitalmodulation signal generating apparatus employing a technique forperforming sensitivity measurement or distortion measurement with highprecision in the case where a base band signal is modulated in anorthogonal manner by means of a carrier signal, and measurement isperformed by employing a signal generating apparatus for generating adigital modulation signal.

DESCRIPTION OF RELATED ART

There is well known as variety of digital signal modulation systems.

In general, as a digital signal modulation system used in a mobilecommunication system or the like, there is known an Orthogonal FrequencyDivision Multiplex modulation system (OFDM).

BRIEF SUMMARY OF THE INVENTION

This OFDM modulation system, as shown in FIG. 10, is directed to asystem of transmitting modulation waves “m” each having a very lowtransmission rate with equal intervals in some tens to some thousands ofbundles.

In order to test a communication device or circuit handling a modulationdigital signal generated by such OFDM modulation system, conventionally,there are employed a digital modulation signal generating apparatus 10configured as shown in FIG. 11 (Refer to Anritsu Technical No. 66<September, 1993>, “MG 3670A Development of Digital Modulation SignalGenerator”, Anritsu Technical No. 74 <October, 1997> “MG 3671B digitalmodulation signal generator”).

This digital modulation signal generating apparatus 10 inputs base bandsignals I and Q outputted from a base band signal generator 11 and acarrier signal C outputted from a carrier signal generator 12 into anorthogonal modulator 13, thereby modulating base band signals I and Q inan orthogonal manner by means of a carrier signal C.

This orthogonal modulator 13 generates a digital modulation signal Sa ofa channel around a carrier frequency “fc”, as shown in FIG. 10.

This digital modulation signal Sa is amplified up to a predeterminedlevel by means of an amplifier 14, and then, the amplification output Sbis inputted to a variable attenuator 15, and is adjusted to a desiredlevel.

The digital modulation signal Sc level adjusted at this variableattenuator 15 is outputted from an output terminal 10 a.

Then, a measurement object 1 such as communication device or circuit isconnected to the output terminal 10 a, wherein the sensitivitycharacteristics or distortion characteristics of the measurement object1 and the like are measured.

For example, in the case of measuring the sensitivity characteristics ofthe communication device, the attenuation quantity of the variableattenuator 15 is set to be large, and a level of the digital modulationsignal Sc outputted from the output terminal 10 a is set to a very lowlevel (for example, −100 dBm), and is inputted to the measurement object1, whereby an error rate of the demodulation signal of the measurementobject 1 is measured.

In addition, in the case of measuring distortion characteristics of acircuit such as an amplifier or mixer, the attenuation quantity of avariable attenuator 15 is set to be small, and a level of the digitalmodulation signal Sc outputted from the output terminal 10 a is set to avery high level (for example, −10 dBm), and is inputted to themeasurement object 1, whereby a spectrum of an output of the measurementobject 1 is observed by means of a spectrum analyzer or the like.

However, as described above, in a digital signal generating apparatus 10for modulating base band signals I and Q by the orthogonal modulator 13in an orthogonal manner by means of a carrier signal, and generating adigital modulation signal, a carrier signal C cannot be completelysuppressed by a slight non-equivalence of the orthogonal modulator 13,and the residual carrier Ca is contained in the generated digitalmodulation signal Sa (this is called a carrier leak).

In the foregoing OFDM modulation system, the power of one modulationwave is as small as 1 of a component of the modulation frequencyrelevant to a total power of all the modulation frequencies. Thus, asshown in FIG. 10, an effect of the residual carrier Ca superimposed onone modulation wave becomes very large.

In the OFDM modulation system, in the case where a level difference α′between such one modulation wave and the residual carrier Ca is small,the modulation precision is worsened.

Thus, when this modulation wave is demodulated at the measurement object1 such as a receiving device, there is a problem that an effect of theresidual carrier Ca appears at a modulation wave, whereby thecharacteristics of the measurement object 1 can not be preciselymeasured.

In order to solve such problem, it is considered that the level of acarrier signal C inputted to the orthogonal modulator 13 is lowered, andconversely, the levels of the base band signals I and Q inputted to theorthogonal modulator 13 are increased, thereby increasing a leveldifference α′.

However, when the level of the carrier signal C inputted to theorthogonal modulator 13 is lowered, there occur problems such asworsened switching characteristics of the orthogonal modulator 13 orlowered gain and the like. Thus, the level of the carrier signal Ccannot be set to a predetermined level or less.

In addition, when the level of the base band signals I and Q inputted tothe orthogonal modulator 13 is increased, there occurs a problem thatthere is increased a mutual modulation distortion (mainly, third highfrequency distortion) that is generated by non-linearity of theorthogonal modulator 13.

Moreover, in this case, the level of the signal inputted to an amplifier14 is increased, and thus, the mutual modulation distortion (mainly,third high frequency distortion) generated by non-linearity of theamplifier 14 increases.

An increase in such generated mutual modulation distortion causes anoccurrence of a problem that the leak power to the adjacent channelsincreases.

Therefore, in a conventional digital modulation signal generatingapparatus, the carrier suppression characteristics of the orthogonalmodulator itself are improved, and the non-linearity of the orthogonalmodulator 13 and amplifier 14 is improved to the maximum.

Hence, these improvements are themselves limited. Therefore, in aconventional digital modulation signal generating apparatus, it isdifficult to make the carrier leak characteristics of the generateddigital modulation signal compatible with mutual modulation distortioncharacteristics at a high level.

DISCLOSURE OF INVENTION

The present invention has been achieved to solve the above-describedconventional technical problems. It is an object of the presentinvention to provide a digital modulation signal generating apparatuscapable of making the carrier leak characteristics of the generateddigital modulation signal compatible with mutual modulation distortioncharacteristics.

In order to achieve the foregoing object, according to one aspect of thepresent invention, there is provided a digital modulation signalgenerating apparatus comprising:

-   -   a base band signal generator (21);    -   a carrier signal generator (25);    -   an orthogonal modulator (24) for generating a digital modulation        signal of a predetermined channel that corresponds to a        frequency of a carrier signal upon receipt of a base band        outputted from the base band signal generator and a carrier        signal outputted from the carrier signal generator;    -   an amplifier (27) for amplifying a digital modulation signal        generated by the orthogonal modulator;    -   an output terminal (20 a) for outputting a digital modulation        signal amplified by the amplifier;    -   first level varying means (22, 23) provided between the base        band signal generator and the orthogonal modulator, for varying        a level of the base band signal, and inputting the level to the        orthogonal modulator;    -   second level varying means (28) provided between the amplifier        and the output terminal, for attenuation varying a level of an        output signal of the amplifier, and outputting the level from        the output terminal;    -   output level specifying means (29) for specifying an output        level value of a digital modulation signal outputted from the        output terminal;    -   judgment means (31) for judging whether or not an output level        value of a digital modulation signal specified by the output        level specifying means is higher than a predetermined value or a        predetermined range; and    -   level diagram switching means (32) for setting the first level        varying means and the second level varying means so that a        digital modulation signal outputted from the output terminal is        set to a predetermined attenuation quantity value which makes        desired carrier leak characteristics compatible with desired        mutual modulation distortion characteristics, respectively,        based on an output level value of a digital modulation signal        specified by the output level specifying means and a judgment        result caused by the judgment means.

In addition, in order to achieve the above object, according to anotheraspect of the present invention, there is provided a digital modulationsignal generating apparatus comprising:

-   -   a base band signal generator (21);    -   a carrier signal generator (25);    -   an orthogonal modulator (24) for generating a digital modulation        signal of a predetermined channel that corresponds to a        frequency of a carrier signal upon the receipt of a base band        signal outputted from the base band signal generator and a        carrier signal outputted from the carrier signal generator;    -   an amplifier (27) for amplifying a digital modulation signal        generated by the orthogonal modulator;    -   an output terminal (20 a) for outputting a digital modulation        signal amplified by the amplifier;    -   first level varying means (22, 23) provided between the base        band signal generator and the orthogonal modulator, for varying        a level of the base band signal, and inputting the level to the        orthogonal modulator;    -   second level varying means provided between the orthogonal        modulator and the amplifier, for attenuation varying a level of        a digital modulation signal outputted from the orthogonal        modulator, and inputting the level to the amplifier;    -   third level varying means (28) provided between the amplifier        and the output terminal, for attenuation varying a level of an        output signal of the amplifier, and outputting the level from        the output terminal;    -   output level specifying means (29) for specifying an output        level value of a digital modulation signal outputted from the        output terminal;    -   judgment means (31) for judging whether or not an output level        value of a digital modulation signal specified by the output        level specifying means is higher than a predetermined value or a        predetermined range; and    -   level diagram switching means (32) for setting the first level        varying means, the second level varying means, and the third        level varying means, respectively, so that a digital modulation        signal outputted from the output terminal is set to a        predetermined attenuation quantity which makes desired carrier        leak characteristics compatible with desired mutual modulation        distortion characteristics based on an output level value of a        digital modulation signal specified by the output level        specifying means and the judgment result caused by a judgment        means.

BRIEF DESCRIPTION OF DRAWINGS

FIG. 1 is a block diagram depicting a configuration of a firstembodiment of a digital modulation signal generating apparatus accordingto the present invention;

FIG. 2 is a block diagram depicting a configuration of essentialportions according to the first embodiment;

FIG. 3A and FIG. 3B are views each showing a level diagram takingcarrier leak characteristics precedence over any other characteristicsaccording to the first embodiment;

FIG. 4 is a view showing a spectrum of an output signal takingprecedence over carrier leak characteristics according to the firstembodiment;

FIG. 5A and FIG. 5B are views each showing a level diagram with mutualmodulation distortion characteristics taking precedence over any othercharacteristics according to the first embodiment;

FIG. 6 is a view showing a spectrum of an output signal, with mutualmodulation distortion characteristics taking precedence over any othercharacteristics according to the first embodiment;

FIG. 7 is a block diagram depicting a configuration of a secondembodiment of a digital modulation signal generating apparatus accordingto the present invention;

FIG. 8A and FIG. 8B are views each showing a level diagram with carrierleak characteristics taking precedence over any other characteristicsaccording to the second embodiment;

FIG. 9A and FIG. 9B are views showing a level diagram with mutualmodulation distortion characteristics taking precedence over any othercharacteristics according to the second embodiment;

FIG. 10 is a view showing a spectrum of a digital signal according to aconventional OFDM modulation system;

FIG. 11 is a block diagram depicting a configuration of a conventionaldigital modulation signal generating apparatus;

FIG. 12 is a view showing a setting table for setting first levelvarying means and second level varying means to a predeterminedattenuation quantity value such that level diagram switching means makesdesired carrier leak characteristics compatible with desired mutuallymodulated characteristics in the first embodiment of the presentinvention; and

FIG. 13 is a view showing a setting table for setting the first levelvarying means, the second level varying means, and the third levelvarying means, respectively to a predetermined attenuation quantityvalue such that level diagram switching means makes desired carrier leakcharacteristics compatible with desired mutually modulatedcharacteristics, in the second embodiment of the present invention.

DETAILED DESCRIPTION OF THE INVENTION

Now, a principle of a digital modulation signal generating apparatusaccording to the present invention will be briefly described here.

In a digital modulation signal generating apparatus of such a type, goodcarrier leak characteristics are required when sensitivitycharacteristics or the like of a measurement object 1 such, as areceiving device, are measured.

That is, in the case of measuring this semiconductor's characteristics,a level of a digital modulation signal inputted to the measurementobject 1 is about −100 dBm, which is very low.

In this way, in measurement in the case where an output level is low,although good carrier leak characteristics are required, mutualmodulation distortion characteristics are not problematic.

In contrast, good mutual modulation distortion characteristics arerequired when measuring distortion characteristics or the like of themeasurement object 1 such as amplifier or mixer is measured.

That is, in the case of measuring distortion characteristics or the likeof this measurement object 1, a level of a digital modulation signalinputted to the measurement object 1 is about −10 dB, which is veryhigh.

In this way, in measurement in the case where an output level is high,although good mutual modulation distortion characteristics are required,carrier leak characteristics are not problematic.

According to the present invention, as described above, attenuation ispaid to the fact that the output level is low when the good carrier leakcharacteristics are required, and the mutual modulation distortioncharacteristics are not problematic; and the fact that the output levelis high when the good mutual modulation distortion characteristics isrequired, carrier leak characteristics are not problematic.

In the present invention, in the case where the output level value of aspecified digital modulation signal is low, even if the mutualmodulation distortion characteristics are worsened, a level diagraminside the apparatus is set so as to improve the carrier leakcharacteristics.

Further in the present invention, in the case where the output levelvalue of a specified digital modulation signal is high, even if thecarrier leak characteristics are worsened, a level diagram inside theapparatus is set so as to improve the mutual modulation distortioncharacteristics.

In this way, in the present invention, the carrier leak characteristicsand mutual modulation distortion characteristics can be compatible witheach other at a high level.

Now, embodiments of the present invention based on the above principlewill be described with reference to accompanying drawings.

(First Embodiment)

FIG. 1 is a block diagram depicting a configuration of a digitalmodulation signal generating apparatus 20 according to a firstembodiment of the present invention, based on the above principle ofoperation.

This digital modulation signal generating apparatus 20 is adopted tooutput a digital modulation signal in accordance with an OFDM modulationsystem from an output terminal 20 a as in the above describedconventional digital modulation signal generating apparatus 10.

First, base band signals Ia and Qa outputted from a base band signalgenerator 21 are attenuated by means of variable attenuators 22 and 23as first level varying means, and are inputted to an orthogonalmodulator 24.

These variable attenuators 22 and 23 can be employed while they areconnected to a variable gain amplifier or a variable attenuator andamplifier in series in the case where a level of the base band signalsIa and Qa outputted from the base band signal generator 21 is low.

In addition, the orthogonal modulator 24 orthogonally modulates baseband signals Ib and Qb that have passed through the variable attenuators22 and 23 by means of a carrier signal C outputted from a carrier signalgenerator 25, and generates a digital modulation signal Sa of a channelthat corresponds to a frequency of the carrier signal.

This orthogonal modulator 24 consists of two mixers 24 a and 24 b,90-degree phase shifter 24 c, and a synthesizer 24 d, as shown in FIG.2, for example.

That is, in this orthogonal modulator 24, the base band signal Ib andcarrier signal C are inputted to the mixer 24 a, and the base bandsignal Qb and a carrier signal C′ phase-shifted by 90 degrees by meansof the phase shifter 24 c are inputted to the mixer 24 b.

Then, output signals of both of the mixers 24 a and 24 b are synthesizedby means of a synthesizer 24 d, and are outputted as a digitalmodulation signal Sa in accordance with an OFDM modulation system.

This digital modulation signal Sa is amplified by means of an amplifier27, and then, the amplified output Sb is inputted to a variableattenuator 28 as second level varying means.

A digital modulation signal Sc subjected to attenuation by means of thisvariable attenuator 28 is outputted from an output terminal 20 a.

This variable attenuator 28 is controlled in attenuation quantity bymeans of a control portion 30 described later in addition to thevariable attenuators 22 and 23.

Output level specifying means 29 is adopted to specify an output levelvalue A of a digital modulation signal outputted from the outputterminal 20 a.

The output level value A specified by this output level specifying means29 is outputted to the control portion 30.

Here, the control portion 30 is composed of a microcomputer includingCPU, ROM, RAM or the like, for example, and the attenuation quantitiesGa and Gb of the variable attenuators 22, 23, and 28 are variablycontrolled according to the output level value A specified by the outputlevel specifying means 29.

This control portion 30 has judgment means 31 and level diagramswitching means 32.

First, the judgment means 31 judges whether or not the output levelvalue A specified by the output level specifying means 29 is higher thana predetermined value (or predetermined range).

Then, in the level diagram switching means 32, based on the output levelvalue A of a digital modulation signal outputted from the outputterminal 20 a specified by the output level specifying means 29 and thejudgment result obtained by the judgment means 31, attenuation quantityof the variable attenuators 22 and 23 as the first level varying meansand the variable attenuator 28 as the second level varying means are setin accordance with the level diagram setting table stored in a ROM asshown in FIG. 12 and the level diagram setting value computed based onthe following formula by means of a CPU, for example, so as to be apredetermined attenuation quantity value which makes desired carrierleak characteristics compatible with desired mutual modulationdistortion characteristics, respectively.

(1) The attenuation quantity of the variable attenuators 22 and 23 is inaccordance with a table shown in FIG. 12.

(2) Attenuation quantity (dB) of variable attenuator 28)=−(attenuationquantity (dB) of variable attenuators 22 and 23)−(specified output levelvalue (dBm))+30

Namely, the level diagram switching means 32 is adopted to switch thelevel diagram inside of the apparatus into a state in which carrier leakcharacteristics takes precedence over mutual modulation distortioncharacteristics and a state in which mutual modulation distortioncharacteristics takes precedence over carrier leak characteristicsaccording to the output level value A of a digital modulation signaloutputted from the output terminal 20 a specified by output levelspecifying means 29 and the judgment result of judgment means 31.

That is, in the case where it is judged that the specified output levelvalue A is lower than a predetermined value (or predetermined range),the control portion 30 set the attenuation quantity of the variableattenuators 22 and 23 to be small so that a level difference between adigital modulation signal of a predetermined channel outputted from theoutput terminal 20 a and the residual carrier is equal to or larger thana predetermined value.

In addition, in this case, the control portion 30 sets an attenuationquantity of the variable attenuator 28 so that a digital modulationsignal of a predetermined channel of the specified output level value Ais outputted from the output terminal 20 a.

In addition, in the case where it is judged that the specified outputlevel value A is higher than a predetermined value (or predeterminedrange), the control portion 30 sets the attenuation quantity of thevariable attenuators 22 and 23 to be large so that a level differencebetween a digital modulation signal of a predetermined channel outputtedfrom the output terminal 20 a and the mutual modulation distortion isequal to or larger than a predetermined value.

In addition, in this case as well, the control portion 30 sets anattenuation quantity of the variable attenuator 28 so that the digitalmodulation signal of a predetermined channel of the specified outputlevel value A is outputted from the output terminal 20 a.

Next, an operation of a digital modulation signal generating apparatus20 will be specifically described by using numeric values.

The presumed numeric value is such that a level of the base band signalsIa and Qa outputted from a base band signal generator 21 is defined as 0dBm, a level of a carrier signal outputted from a carrier signalgenerator 25 is defined as 10 dBm, a carrier suppression ratio of theorthogonal modulator 24 is defined as −70 dB, a gain of the orthogonalmodulator 24 (a difference between a level of base band signals Ib andQb and a level of an output signal Sa) is defined as 10 dB, and a gainof the amplifier 27 is defined as 20 dB.

In addition, the mutual modulation distortion generated by theorthogonal modulator 24 is very low when its input signal level is lowerthan −20 dBm, and is gradually worsened in excess of −20 dB.

Further, the mutual modulation distortion generated by the amplifier 27is very low when its input signal level is lower than −10 dBm, and isgradually worsened in excess of −10 dBm.

Furthermore, the judgment means 31 judges whether or not the outputlevel value A specified from the output level specifying means 29 ishigher than −50 dBm which is a predetermined value.

Here, in order to perform sensitivity measurement or the like of ameasurement object 1 such as a receiving device, assume that the outputlevel value A is specified as −100 (dBm), for example, by the outputlevel specifying means 29.

At this time, the judgment means 31 judges that the specified outputlevel value A is lower than −50 (dBm) that is a predetermined value.

Still furthermore, a level difference (hereinafter, referred to as acarrier leak ratio) between a total level of a digital modulation signalrequired during measurement of such sensitivity characteristics and theresidual carrier is 50 dB or more.

That a level difference between a total level and the residual carrieris secured to be 50 dB or more denotes that, in OFDM modulation of 1,000modulation waves, for example, a level of one modulation wave is 1/1,000of the total level, and thus, a level difference from one modulationwave superimposed on the residual carrier is ensured by 20 dB or more.

In addition, a difference between a total level of a digital modulationsignal required for measurement of distortion characteristics such as anamplifier or mixer and the like and a total level of a mutual modulationdistortion (hereinafter, referred to as adjacent channel leak ratio) is70 dB or more.

Based on the above presumed numeric value, the level diagram switchingmeans 32 sets the level diagram inside of the apparatus so that a leveldifference between a digital modulation signal of a predeterminedchannel outputted from the output terminal 20 a and the residual carrieris increased, and the digital modulation signal of the output levelvalue A specified by the output level specifying means 29 is outputtedfrom the output terminal 20 a.

First, in the level diagram switching means 32, as shown in FIG. 3A andFIG. 3B, the attenuation quantity Ga of the variable attenuators 22 and23 is set to 10 dB which is comparatively small, whereby the input levelof the base band signals I and Q relevant to the orthogonal modulator 24is set to −10 dBm.

In the level diagram shown in FIG. 3A and FIG. 3B, the digitalmodulation signal, mutual modulation distortion, and residual carrierare shown as a total level.

At this time, a total level of the digital modulation signal Saoutputted from the orthogonal modulator 24 is 0 dBm (=−10+10), and alevel of the residual carrier Ca included in this signal is −60 dBm(=10−70).

This digital modulation signal Sa is amplified to 20 dBm by means of theamplifier 27, and the amplified output Sb is inputted to the variableattenuator 28.

At this time, the level diagram switching means 32 sets the attenuationquantity Gb of the variable attenuator 28 to 120 dB (=20−(−100)),whereby a level of the digital modulation signal Sc outputted from theoutput terminal 20 a is set to −100 dBm that corresponds to thespecified output level value A.

In this case, the residual carrier Ca as well as digital modulationsignal Sa is amplified to −40 dBm by means of the amplifier 27, andattenuation of 120 dB is received by the variable attenuator 28. Thus,the residual carrier Cc attenuated to −160 dBm is outputted from theoutput terminal 20 a.

In this manner, a carrier leak ratio α relevant to a total level at theoutput terminal 20 a is 60 dB (=−100−(−160)), and a carrier leak ratioα′ (See FIG. 4) relevant to a level of a modulation wave superimposed onthe residual carrier Cc is 30 dB as shown in FIG. 4.

As a result, a digital modulation signal in excess of a carrier leakratio of 50 dB relevant to a total level required for this measurementcan be obtained.

Of the level diagrams shown in FIG. 3A and FIG. 3B, the level diagramfrom the base band signal generator 21 to an output of the amplifier 27is such that the output level value A specified by the output levelspecifying means 29 does not change in a range which is lower than −50(dBm), and changes according to the output level value A specified bythe attenuation quantity of the variable attenuator 28.

Namely, in this range, the carrier leak ratio α relevant to a totallevel is ensured as 60 dB.

In this way, in the case where the output level value A specified by theoutput level specifying means 29 is lower than a predetermined value,the level of the residual carrier Cc is very low relevant to the levelof the digital modulation signal Sc outputted from the output terminal20 a, as shown in FIG. 4, by the level diagram switching means 32.

Therefore, in this case, the sensitivity measurement relevant to themeasurement object 1 such as a receiving device can be preciselyperformed without being affected by the residual carrier Cc.

As shown in FIG. 3A and FIG. 3B, in the level diagrams taking carrierleak characteristics precedence over any other characteristics, theinput signal level of the orthogonal modulator 24 is higher than −20dBm.

Thus, a mutual modulation distortion Ra is generated at a comparativelyhigh level (−50 dBm) with an output of the orthogonal modulator 24.

Moreover, the input signal level of the amplifier 27 is higher than −10dBm, and thus, a mutual modulation distortion Rb of a level (−20 dBm)higher than such a gain is generated.

Hence, this mutual modulation distortion Rb is attenuated by means ofthe variable attenuator 28, and a mutual modulation distortion Rc of−140 dBm is outputted from the output terminal 20 a.

Therefore, the adjacent channel leak ratio β in this level diagram is 40dB (=−100−(−140)).

This mutual modulation distortion Rc leaks not only into a predeterminedchannel and but also into the adjacent channels to this predeterminedchannel, as shown in FIG. 4.

Hence, a level difference β′ between individual modulation wave “m” andeach distortion component superimposed on the wave is ensured by 40 dB,and the modulation precision is hardly affected.

Thus, the influence of a receiving device or the like upon themeasurement object 1 in sensitivity measurement can be almost ignored.

In addition, in the case of performing distortion measurement or thelike of the measurement object 1 such as an amplifier or mixer, theoutput level value A is specified as −10 (dBm), for example, by theoutput level specifying means 29.

At this time, judgment means 31 judges that the output level value Aspecified by −50 (dBm) that is a predetermined value is high.

Thus, the level diagram switching means 32 sets the level diagram insideof the apparatus so that a mutual modulation distortion is reduced, andthe digital modulation signal of the specified level value A isoutputted from the output terminal 20 a.

That is, as in the level diagrams shown in FIG. 5A and FIG. 5B, theattenuation quantity of the variable attenuators 22 and 23 is set to 30dB, whereby the input level of the base band signals Ib and Qb relevantto the orthogonal modulator 24 is set to −30 dBm.

At this time, a total level of the digital modulation signal Saoutputted from the orthogonal modulator 24 is set to −20 dBm (=−30+10),and a level of the mutual modulation distortion Ra is −100 dBm, forexample.

This digital modulation signal Sa is amplified to 0 dBm by means of theamplifier 27, and the amplified output Sb is inputted to the variableattenuator 28.

At this time, the level diagram switching means 32 sets the attenuationquantity of the variable attenuator 28 to 10 dB (=0−(−10)), whereby thelevel of the digital modulation signal Sc outputted from the outputterminal 20 a is set to −10 dBm that corresponds to the specified levelA.

Here, the level of the digital modulation signal Sa inputted to theamplifier 27 is lower than −10 dBm, and thus, an increase in mutualmodulation distortion caused by the amplifier 27 can be almost ignored.

In this manner, from the amplifier 27, there is generated a mutualmodulation distortion Rb of −80 dBm obtained by amplifying the mutualmodulation distortion Ra by 20 dB.

This mutual modulation distortion is attenuated by 10 dB by means of thevariable attenuator 28, whereby a mutual modulation distortion Rc of −90dBm is generated at the output terminal 20 a.

Therefore, the adjacent channel leak ratio β at the output terminal 20 ais 80 dB (=−10−(−90)), and there can be obtained a digital modulationsignal when the adjacent channel leak ratio required for thismeasurement exceeds 70 dB.

Of the level diagrams shown in FIG. 5A and FIG. 5B, a level diagram fromthe base band signal generator 21 to an output of the amplifier 27 doesnot change in a range in which the output level value A specified by theoutput level specifying means 29 is higher than −50 (dBm).

That is, in this range, only the attenuation quantity of the variableattenuator 28 changes according to the specified level value A, and theadjacent channel leak ratio β is ensured by 80 dB.

In this way, in the case where the output level value A specified by theoutput level specifying means 29 is higher than a predetermined value, adifference β′ between a level of each modulation wave “m” of the digitalmodulation signal Sc outputted from the output terminal 20 a and a levelof the mutual modulation distortion Rc that leaks into a predeterminedchannel and the adjacent channel is ensured by 80 dB by the leveldiagram switching means 32, as shown in FIG. 6.

Therefore, in this case, the measurement of characteristics of anamplifier or mixer and the like relevant to the measurement object 1 canbe precisely performed without being affected by the mutual modulationdistortion Rc.

As shown in FIG. 5A and FIG. 5B, in the level diagram with the mutualmodulation distortion characteristics taking precedence over any othercharacteristics, the input signal level of the orthogonal modulator 24is as low as −30 dBm.

Therefore, at the output terminal 20 a, the carrier leak ratio α′relevant to a level of a modulation wave superimposed on the residualcarrier is worsened as 10 dB, and the residual carrier Cc at itscomparatively large level is generated as shown in FIG. 6.

Hence, the carrier leak ratio α relevant to a total level is ensured by40 dB, and thus, the level of the residual carrier Cc is sufficientlysmaller than a total level.

As a result, as in measurement of distortion characteristics of theamplifier or mixer or the like relevant to the measurement object 1, inmeasurement concerning a signal total level, an effect of the residualcarrier Cc can be almost ignored.

(Second Embodiment)

In the meantime, in the digital modulation signal generating apparatus20 according to the first embodiment, the input signal level of theorthogonal modulator 24 and amplifier 27 is determined by means of thevariable attenuators 22 and 23 provided at the front stage of theorthogonal modulator 24.

Hence, in a digital modulation signal generating apparatus 20′ accordingto the second embodiment, as shown in FIG. 7, a variable attenuator 26as second level varying means is provided between the orthogonalmodulator 24 and the amplifier 27, as shown in FIG. 7.

This variable attenuator 26 is controlled by the level diagram switchingmeans 32 together with the variable attenuators 22 and 23 as the firstlevel varying means and the variable attenuator 28 as third levelvarying means.

In this manner, the variable attenuator 26 is provided at the frontstage of the amplifier 27, whereby the variable attenuators 22 and 23can be used independently for input level variation of the orthogonalmodulator 24 and the variable attenuator 26 can be used for input levelvariation of the amplifier 27. Thus, the level control corresponding toeach of the dynamic ranges of the orthogonal modulator 24 and theamplifier 27 can be performed.

For example, in the digital modulation signal generating apparatus 20according to the first embodiment, when the dynamic range of theamplifier 27 is narrower than that of the orthogonal modulator 24, therange is restricted to the dynamic range of the amplifier 27, and thecarrier leak ratio cannot be increased because the input level of theorthogonal modulator 24 cannot be increased.

Hence, in the digital modulation signal generating apparatus 20′according to the second embodiment, the variable attenuator 26 isprovided at the front stage of the amplifier 27, whereby the carrierleak ratio can be further increased.

In the level diagram switching means 32, based on the output level valueof the digital modulation signal specified by the output levelspecifying means 29 and the judgment result caused by the judgment means31, the attenuation quantities Ga, Gb, and Gc of the variableattenuators 22 and 23 as the first level varying means, the variableattenuator 26 as second level means, and the variable attenuator 28 asthird level varying means are set in accordance with the level diagramsetting table stored in a ROM as shown in FIG. 13, for example, and thelevel diagram setting value computed based on the following formula bythe CPU so as to be a predetermined attenuation quantity value whichmakes compatible desired carrier leak characteristics and desired mutualmodulation distortion characteristics.

(1) The attenuation quantity of the variable attenuators 22 and 23 andthe variable attenuator 26 is in accordance with the table shown in FIG.13.

(2) Attenuation quantity (dB) of variable attenuator 28=−(attenuationquantity (dB) of variable attenuators 22 and 23)−(attenuation quantity(dB) of variable attenuator 26)−(specified output level value dBm)+30

Now, an operation of a digital orthogonal modulator signal generatingapparatus 20′ according to the second embodiment will be specificallydescribed by employing numeric values.

In the digital modulation signal generating apparatus 20′ according tothe second embodiment, a presumed numeric value is selected as in thedigital modulation signal generating apparatus 20 according to the firstembodiment.

That is, the presumed numeric value is such that a level of the baseband signals Ia and Qa outputted from the base band signal generator 21is defined as 0 dBm; a level of a carrier signal outputted from thecarrier signal generator 25 is defined as 10 dBm; a carrier suppressionratio of the orthogonal modulator 24 is defined as −70 dB; a gain of theorthogonal modulator 24 (a difference between a level of the base bandsignals Ib and Qb and a level of an output signal Sa) is defined as 10dB; and a gain of the amplifier 27 is defined as 20 dB.

In addition, the mutual modulation distortion generated by theorthogonal modulator 24 is very low when its input signal level is lowerthan −20 dBm, and is gradually worsened in excess of −20 dBm.

Further, the mutual orthogonal modulator distortion generated by theamplifier 27 is very low when its input signal level is lower than −10dBm, and is gradually worsened in excess of −10 dBm.

The judgment means 31 judges whether or not the level value A specifiedfrom the output level specifying means 29 is higher than −50 dBm that isa predetermined value.

Here, in order to perform sensitivity measurement or the like of ameasurement object 1 such as a receiving device, the output level valueA is specified as −100 (dBm), for example, by the output levelspecifying means 29.

At this time, the judgment means 31 judges that the output level value Aspecified by −50 (dBm) that is a predetermined value is low.

The level diagram switching means 32 sets the level diagram inside ofthe apparatus so that the carrier leak ratio is large, and a digitalmodulation signal of the specified output level value A is outputtedfrom the output terminal 20 a.

First, the level diagram switching means 32 sets the attenuationquantity Ga of the variable attenuators 22 and 23 to 0 dB, as shown inFIG. 8A and FIG. 8B, and set to 0 dBm an input level of the base bandsignals Ib and Qb relevant to the orthogonal modulator 24.

At this time, a total level of the digital modulation signal Saoutputted from the orthogonal modulator 24 is set to 10 dBm, and a levelof the residual carrier Ca is set to −60 dBm.

The digital modulation signal Sa and the residual carrier Ca areinputted to the variable attenuator 26.

At this time, the level diagram switching means 32 sets the attenuationquantity Gb of the variable attenuator 26 to 20 dB, whereby the digitalmodulation signal Sa is attenuated to −10 dBm, the residual carrier Cais attenuated to −80 dBm, and these signal and carrier are inputted tothe attenuator 27.

The digital modulation signal Sb and residual carrier Cb attenuated bymeans of this variable attenuator 26 are amplified to 10 dBm and −70dBm, respectively, by means of the attenuator 27, and the amplifiedoutputs Sc and Cc are inputted to the variable attenuator 28.

At this time, the level diagram switching means 32 sets the attenuationquantity Gc of the variable attenuator 28 to 110 dB, and sets a level ofthe digital modulation signal Sd outputted from the output terminal 20 ato −100 dBm that corresponds to the specified output level value A,whereby the level of the residual carrier Cd outputted from the outputterminal 20 a is attenuated to −170 dBm.

Therefore, a carrier leak ratio α relevant to a total level at theoutput terminal 20 a is set to 70 dB, and a carrier leak ratio α′ (seeFIG. 4) relevant to a level of the modulation waves superimposed on theresidual carrier is set to 40 dB a shown in FIG. 10.

In this manner, in the digital modulation signal generating apparatus20′ according to the second embodiment, there can be obtained thecarrier leak ratio which is even higher than that of the digitalmodulation signal generating apparatus 20 according to the firstembodiment. Thus, the sensitivity measurement relevant to themeasurement object 1 such as receiving device can be performed moreprecisely.

In this way, in the level diagram of FIG. 8A and FIG. 8B in whichcarrier leak characteristics take precedence over any othercharacteristics, the input signal level of the orthogonal modulator 24is higher, and the mutual modulation distortion Ra is generated at ahigher level (−35 dBm).

Hence, the input signal level of the attenuator 27 is lowered to −10 dBmby means of the variable attenuator 27. Thus, the generation of themutual modulation distortion caused by the attenuator 27 itself can bealmost ignored.

In this manner, the mutual modulation distortion Rc of the output of theattenuator 27 can be set to −30 dBm, which is lower than a case of thedigital modulation signal generating apparatus 20.

As a result, the adjacent channel leak ratio β at the output terminal 20a is set to 40 dB in the same manner as in the digital modulation signalgenerating apparatus 20, and an extreme increase in mutual modulationdistortion caused by an input level increase can be suppressed.

In addition, in the case of performing distortion measurement or thelike of the measurement object 1 such as amplifier or mixer, the outputlevel value A is specified as −10 (dBm) by the output level specifyingmeans 29.

At this time, the judgment means 31 judges that the level value Aspecified by −50 (dBm) that is a predetermined value is high.

The level diagram switching means 32 sets a level diagram inside of theapparatus so that the mutual modulation distortion is reduced, and adigital modulation signal of the specified output level value A isoutputted from the output terminal 20 a.

First, the level diagram switching means 32 sets the attenuationquantity Ga of the variable attenuators 22 and 23 to 30 dB, as shown inFIG. 9A and FIG. 9B, whereby the input level of the base band signals Iband Qb relevant to the orthogonal modulator 24 is set to −30 dBm.

At this time, the total level of the digital modulation signal Saoutputted from the orthogonal modulator 24 is set to −20 dBm, and themutual modulation distortion Ra of the orthogonal modulator 24 is set to−100 dBm, similarly.

This digital modulation signal Sa and mutual modulation distortion Raare inputted to the variable attenuator 26.

At this time, the level diagram switching means 32 sets the attenuationquantity of the variable attenuator 26 to 0 dB, whereby the − digitalmodulation signal Sb of 20 dB and the residual carrier Cb of −100 dBmare inputted to the amplifier 27.

In the level diagrams shown in FIG. 8A and FIG. 8B, an input signallevel of the orthogonal modulator 24 is higher, and thus, the mutualmodulation distortion Ra is generated at a higher level (−35 dBm).

Hence, the input signal level of the amplifier 27 is reduced to −10 dBmby means of the variable attenuator 26. Thus, the generation of a mutualmodulation distortion caused by this amplifier 27 itself can be almostignored.

In addition, the mutual modulation distortion Rc of an output of theamplifier 27 can be reduced to −30 dBm, which is lower than a case ofthe signal generating apparatus 20.

As a result, the adjacent channel leak ratio β at the output terminal 20a is set to 40 dB in the same manner as in the case of the digitalmodulation signal generating apparatus 20, and an extreme increase inmutual modulation distortion caused by an input level increase can berestricted.

In addition, in the case of performing distortion measurement or thelike of the measurement object 1 such as an amplifier or mixer, theoutput level value A is specified as −10 (dBm) by the output levelspecifying means 29.

At this time, the judgment means 31 judges that the output level value Aspecified by −50 (dBm) that is a predetermined value is high.

The level diagram switching means 32 sets the level diagram inside ofthe apparatus so that the mutual modulation distortion is reduced, andthe digital modulation signal of the specified output level value A isoutputted from the output terminal 20 a.

First, the level diagram switching means 32 sets the attenuationquantity Ga of the variable attenuators 22 and 2 to 30 dB, as shown inFIG. 9A and FIG. 9B, whereby the input level of the base band signals Iband Qb relevant to the orthogonal modulator 24 is set to −30 dBm.

At this time, a total level of the digital modulation signal Saoutputted from the orthogonal modulator 24 is set to −20 dBm, and themutual modulation distortion Ra of the orthogonal modulator 24 is set to−100 dBm in the same way as previously.

The digital modulation signal Sa and the mutual modulation distortion Raare inputted to the variable attenuator 26. At this time, the leveldiagram switching means 32 sets the attenuation quantity of the variableattenuator 26 to 0 dB, and the digital modulation signal Sb of −20 dBmand the residual carrier Cb of −100 dBm are inputted to the amplifier27.

This digital modulation signal Sb is amplified to 0 dBm by means of theamplifier 27, and its amplified output Sc is inputted to the variableattenuator 28.

In addition, as described previously, the input signal level of theamplifier 27 is as low as −20 dBm, and thus, the mutual modulationdistortion generated by the amplifier 27 itself can be ignored. From theamplifier 27, the mutual modulation distortion Rc of −80 dBm isoutputted, and is inputted to the variable attenuator 28.

At this time, the level diagram switching means 32 sets the attenuationquantity Gc of the variable attenuator 28 to 10 dB, and the level of thedigital modulation signal Sd outputted from the output terminal 20 a isset to −10 dBm that corresponds to the specified output level value A.

In addition, the mutual modulation distortion Rd outputted from theoutput terminal 20 a is set to −90 dBm due to the attenuation caused bythe variable attenuator 28.

Therefore, the adjacent channel leak ratio β at the output terminal 20 ais set to 100 dB in the same way as previously, and measurement of thedistortion characteristics of an amplifier or mixer and the likerelevant to the measurement object 1 can be precisely performed withoutbeing affected by this mutual modulation distortion.

In the level diagram shown in FIG. 9A and FIG. 9B with the mutualmodulation distortion characteristics taking precedence over any othercharacteristics, the input signal level of the orthogonal modulator 24is low, and thus, the levels of the residual carriers Ca, Cb, Cc, and Cdare high. However, the carrier leak ratio α relevant to a total level isensured by 40 dB. In the same manner as previously, an effect ofmeasurement of distortion characteristics relevant to the measurementobject 1 such as amplifier or mixer with respect to a signal total levelcan be almost ignored.

In the foregoing digital modulation signal generating apparatuses 20 and20′, the judgment means 31 judges whether or not the output level valueA specified from the output level specifying means 29 is higher than apredetermined value (−50 dBm) so that the level diagram in the apparatusis varied.

Hence, for example, in the case where a range of −40 to −60 (dBm) isdefined as a predetermined range, and the specified output level value Ais lower than this predetermined range, there may be employed a leveldiagram with carrier leak characteristics taking precedence over anyother characteristics in the same way as previously. In the case wherethe above range is higher than the predetermined range, there may beemployed a level diagram with mutual modulation distortioncharacteristics taking precedence over any other characteristics in thesame manner as previously.

In this case, when the specified output level value A is within apredetermined range, the attenuation quantity (or gain) of the variableattenuators 22 and 23 in the case of the digital modulation signalgenerating apparatus 20 and the attenuation quantity (or gains) of thevariable attenuators 22 and 23, and the attenuation quantity of thevariable attenuator 26 in the case of digital modulation signalgenerating apparatus 20′ are set to values between a value obtained whencarrier leak characteristics precedes any other characteristics and avalue obtained when mutual modulation distortion characteristicsprecedes any other characteristics, whereby the carrier leakcharacteristics and mutual modulation distortion characteristics arecontrolled to be placed in a comparatively good state.

As has been described above, according to the first aspect of thepresent invention, there is provided a digital modulation signalgenerating apparatus, wherein first level varying means is providedbetween a base band signal generator and an orthogonal modulator; secondlevel varying means is provided between an amplifier and an outputterminal; it is judged whether or not an output level value specified byoutput level specifying means is higher than a predetermined value or apredetermined range; when it is judged that the specified output levelvalue is lower than the predetermined value or the predetermined range,the first level varying means is set so that a level difference betweena digital modulation signal outputted from an output terminal and aresidual carrier is equal to or larger than a predetermined value, andan attenuation quantity of the second level varying means is set so thata digital modulation signal of the specified output level value isoutputted from an output terminal; when the specified output level valueis higher than the predetermined value or the predetermined range, thefirst level varying means is set so that a level difference between adigital modulation signal outputted from an output terminal and a mutualmodulation distortion is equal to or larger than the predetermined valueor range; and an attenuation quantity of the second level varying meansis set so that a digital modulation signal of the specified output levelvalue is outputted from an output terminal.

Thus, in the case where a low output level is specified in order toperform sensitivity measurement, such as a receiving device, a digitalmodulation signal can be output such that carrier leak characteristicssuitable to such sensitivity measurement are very good. In the casewhere a high output level is specified in order to perform distortionmeasurement such as an amplifier or mixer, a digital modulation signalcan be output such that mutual modulation distortion characteristicssuitable to the distortion characteristics or the like are very good.Thus, carrier leak characteristics and mutual modulation distortioncharacteristics can be compatible with each other at a high level.

According to the second aspect of the present invention, there isprovided a digital modulation signal generating apparatus, wherein firstlevel varying means is provided between a base band signal generator andan orthogonal modulator; second level varying means is provided betweenan orthogonal modulator and a modulator; third level varying means isprovided between an amplifier and an output terminal; it is judgedwhether or not an output level value specified by output levelspecifying means is higher than a predetermined value or a predeterminedrange; when it is judged that the specified output level value is lowerthan the predetermined value or predetermined range, the first levelvarying means is set so that a level difference between a digitalmodulation signal outputted from an output terminal and a residualcarrier is equal to or larger than the predetermined value or range, andthe second level varying means and third level varying means are set sothat the digital modulation signal of the specified output level valueis outputted from the output terminal; and when it is judged that thespecified output level value is higher than the predetermined value orpredetermined range, the first level varying means and second levelvarying means are set so that a level difference between the digitalmodulation signal outputted from the output terminal and the mutualmodulation distortion is equal to or larger than the predetermined valueor range, and the third level varying means is set so that the digitalmodulation signal of the specified output level value is outputted fromthe output terminal.

Thus, in the case where a low output level is specified in order toperform sensitivity measurement such as a receiving device, a digitalmodulation signal can be outputted such that carrier leakcharacteristics suitable to the sensitivity measurement or the like arevery good. In the case where a high output level is specified in orderto perform distortion measurement such as an amplifier or mixer, adigital modulation signal can be outputted such that mutual modulationdistortion characteristics suitable to those distortion characteristicsor the like are very good. Moreover, there are provided independentlevel varying means at the front stage of each of the orthogonalmodulator and amplifier. Thus, the input signal level of the orthogonalmodulator can be increased without being limited by the amplifiercharacteristics, and the carrier leak characteristics and mutualmodulation distortion characteristics can be compatible with each otherat a high level.

Therefore, according to the present invention as has been describedabove, the problems in the prior art are solved, and there can beprovided a digital modulation signal generating apparatus capable ofmaking compatible with each other the carrier leak characteristics ofthe generated digital modulation signal and the mutual modulationdistortion characteristics at a high level.

1. A digital modulation signal generating apparatus, comprising: a bandsignal generator which generates a base band signal; a carrier signalgenerator which generates a carrier signal; an orthogonal modulatorwhich generates a digital modulation signal of a predetermined channelthat corresponds to a frequency of the carrier signal upon receipt ofthe base band signal generated by the base band signal generator and thecarrier signal generated by the carrier signal generator; an amplifierwhich amplifies the digital modulation signal generated by theorthogonal modulator; an output terminal which outputs the digitalmodulation signal amplified by the amplifier; a first level varyingmember provided between the base band signal generator and theorthogonal modulator, which varies a level of the base band signal, andinputs the base band signal which has had the level varied to theorthogonal modulator; a second level varying member provided between theamplifier and the output terminal, which attenuates and varies a levelof an output signal output from the amplifier, and outputs the outputsignal which has had the level attenuated and varied from the outputterminal; output level specifying member which specifies an output levelvalue of the digital modulation signal output from the output terminal;a judgment section which judges whether or not the output level value ofthe digital modulation signal specified by the output level specifyingmember is higher than a predetermined value or a predetermined range;and a level diagram switching section which sets the first level varyingmember and the second level varying member so that the digitalmodulation signal output from the output terminal is a predeterminedattenuation quantity value which makes desired carrier leakcharacteristics compatible with desired mutual modulation distortioncharacteristics, respectively, based on the output level value of thedigital modulation signal specified by the output level specifyingmember and a judgment result caused by the judgment member.
 2. A digitalmodulation signal generating apparatus according to claim 1, whereinsaid level diagram switching member is adopted to switch a level diagraminside of the apparatus between a state in which carrier leakcharacteristics precedes mutual modulation distortion characteristicsand a state in which mutual modulation distortion characteristicsprecedes carrier leak characteristics according to the output levelvalue of the digital modulation signal output from said output terminalspecified by said output level specifying member and a judgment resultof said judgment member.
 3. A digital modulation signal generatingapparatus according to claim 2, wherein said level diagram switchingsection is adopted to set an attenuation quantity of said first levelvarying member to be small so that a level difference between thedigital modulation signal of the predetermined channel output from saidoutput terminal and a residual carrier is equal to or larger than apredetermined value in the case where the output level value specifiedby said output level specifying member is lower than the predeterminedvalue or predetermined range by said judgment section, and to set anattenuation quantity of said second level varying member so that thedigital modulation signal of the predetermined channel of the outputlevel value specified by said output level specifying member is outputfrom said output terminal.
 4. A digital modulation signal generatingapparatus according to claim 2, wherein said level diagram switchingsection is adopted to set an attenuation quantity of said first levelvarying member to be small so that a level difference between thedigital modulation signal of the predetermined channel output from saidoutput terminal and a mutual modulation distortion is equal to or largerthan a predetermined value when it is judged that an output level valuespecified by said output level specifying member is higher than apredetermined value or a predetermined range by said judgment section,and to set an attenuation quantity of said second level varying memberso that the digital modulation signal of the predetermined channel ofthe output level value specified by said output level specifying memberis output from said output terminal.
 5. A digital modulation signalgenerating apparatus according to claim 2, wherein said level diagramswitching section is adopted to set said first level varying member sothat a level difference between the digital modulation signal outputfrom said output terminal and a residual carrier contained therein isequal to or larger than a predetermined value or range by said judgmentsection when it is judged that an output level value specified by saidoutput level specifying member is lower than the predetermined value orpredetermined range; to set said second level varying member so that thedigital modulation signal of said specified level value is output fromsaid output terminal; to set said first level varying member so that alevel of the digital modulation signal output from said output terminaland a level of the mutual modulation distortion contained therein areequal to or larger than a predetermined value when it is judged thatsaid specified level value is higher than said predetermined value orsaid predetermined range; and to set said second level varying membermeans so that a digital modulation signal of said specified level valueis output from said output terminal.
 6. A digital modulation signalgenerating apparatus according to claim 1, wherein said digitalmodulation signal generating apparatus further comprises a computationsection which computes a predetermined attenuation quantity value forsaid first level varying member and said second level varying member setby said level diagram switching section based on the output level valueof the digital modulation signal specified by at least said output levelspecifying member and the judgment result caused by said judgmentsection.
 7. A digital modulation signal generating apparatus accordingto claim 1, wherein said digital modulation signal generating apparatusfurther comprises a storage section which stores in advance in a tableformat a predetermined attenuation quantity value for the first levelvarying member and the second level varying member set by said leveldiagram switching section.
 8. A digital signal generating apparatuscomprising: a base band signal generator which generates a base bandsignal; a carrier signal generator which generates a carrier signal; anorthogonal modulator which generates a digital modulation signal of apredetermined channel that corresponds to a frequency of the carriersignal upon the receipt of the base band signal generated by the baseband signal generator and the carrier signal generated by the carriersignal generator; an amplifier which amplifies the digital modulationsignal generated by the orthogonal modulator; an output terminal whichoutputs the digital modulation signal amplified by the amplifier; afirst level varying member provided between the base band signalgenerator and the orthogonal modulator, which varies a level of the baseband signal, and inputs the base band signal which has had the levelvaried to the orthogonal modulator; a second level varying memberprovided between the orthogonal modulator and the amplifier, whichattenuates and varies a level of the digital modulation signal outputtedfrom the orthogonal modulator, and inputs the digital modulation signalwhich has had the level attenuated and varied to the amplifier; a thirdlevel varying member provided between the amplifier and the outputterminal, which attenuated and varied a level of an output signal outputfrom the amplifier, and outputs the output signal which has had thelevel attenuated and varied from the output terminal; an output levelspecifying member which specifies an output level value of the digitalmodulation signal output from the output terminal; a judgment sectionwhich judges whether or not the output level value of the digitalmodulation signal specified by the output level specifying member ishigher than a predetermined value or a predetermined range; and a leveldiagram switching section which sets the first level varying member,said second level varying member, and said third level varying member toa predetermined attenuation value, respectively, so that the digitalmodulation signal output from the output terminal makes desired carrierleak characteristics compatible with desired mutual modulationdistortion characteristics based on the output level value of thedigital modulation signal specified by the output level specifyingmember and a judgment result caused by the judgment section.
 9. Adigital modulation signal generating apparatus according to claim 8,wherein said level diagram switching section is adopted to switch alevel diagram inside of the apparatus between a state in which carrierleak characteristics precedes mutual modulation distortioncharacteristics and a state in which mutual modulation distortioncharacteristics precedes carrier leak characteristics according to theoutput level value of the digital modulation signal output from saidoutput terminal specified by said output level specifying section andthe judgment result of said judgment section.
 10. A digital modulationsignal generating apparatus according to claim 8, wherein said leveldiagram switching section is adopted to set an attenuation quantity ofsaid first level varying member to be small so that a level differencebetween the digital modulation signal of the predetermined channeloutput from said output terminal and a residual carrier is equal to orlarger than a predetermined value when it is judged that the outputlevel value specified by said output level specifying member is lowerthan the predetermined value or the predetermined range by said judgmentsection, to set an automation quantity of said second level varyingmember to be large so that a level difference between the digitalmodulation signal of the predetermined channel output from said outputterminal and a residual carrier is equal to or larger than apredetermined value, and to set an attenuation quantity of said thirdlevel varying member so that the digital modulation signal of thepredetermined channel of the output level value specified by said outputlevel specifying section is output from said output terminal.
 11. Adigital modulation signal generating apparatus according to claim 8,wherein said level diagram switching section is adopted to set anattenuation quantity of said first level varying member to be large sothat a level difference between the digital modulation signal of thepredetermined channel output from said output terminal and a mutualmodulation distortion is equal to or larger than a predetermined valuewhen it is judged that an output level value specified by said outputlevel specifying member is higher than the predetermined value or thepredetermined range by said judgment section, to set an attenuationquantity of said second level varying member to be large so that a leveldifference between the digital modulation signal of the predeterminedchannel output from said output terminal and a mutual modulationdistortion is equal to or larger than a predetermined value, and to setan attenuation quantity of said third level varying member so that thedigital modulation signal of the predetermined channel of the outputlevel value specified by said output level specifying member is outputfrom said output terminal.
 12. A digital modulation signal generatingapparatus according to claim 8, wherein said level diagram switchingsection is adopted to set said first level varying member section andsaid second level varying member so that a level difference between thedigital modulation signal output from said output terminal and a mutualmodulation distortion is equal to or larger than a predetermined valuewhen it is judged that an output level value specified by said outputlevel specifying member is lower than the predetermined value or thepredetermined range; to set said third level varying member so that thedigital modulation signal of said specified level value is output fromsaid output terminal; to set said first level varying member and saidsecond level varying member so that a level difference between thedigital modulation signal output from said output terminal and themutual modulation distortion contained therein is equal to or largerthan a predetermined value when it is judged that said specified levelvalue is higher than said predetermined value or said predeterminedrange; and to set said third level varying member so that the digitalmodulation signal of said specified level value is output from saidoutput terminal.
 13. A digital modulation signal generating apparatusaccording to claim 8, wherein said digital modulation signal generatingapparatus further comprises a computation section which computes apredetermined attenuation quantity value for said first level varyingmember, said second level varying member, and said third level varyingmember set by said level diagram switching section based on the outputlevel value of the digital modulation signal specified by at least saidoutput level specifying member and the judgment result caused by saidjudgment section.
 14. A digital modulation signal generating apparatusaccording to claim 8, wherein said digital modulation signal generatingapparatus further comprises a storage section which stores in advance ina table format a predetermined attenuation quantity value for the firstlevel varying member, the second level varying member, and the thirdlevel varying member set by said level diagram switching section.